NASA solar system facts - Comet

A comet is a collection of gas and dust a few miles across.
Normally it is far too faint to be seen even with a powerful
telescope. But when it approaches the Sun it becomes brighter, and
can then be seen with a telescope. Very occasionally a comet becomes
bright enough to be seen with the unaided eye.

NASA solar system facts:
a comet appears as a bright ball of
light. When close to the Sun it may have a long, glowing tail. The
Sun's rays cause the gas contained in the comet to glow and spread
out to form the tail. Night after night the comet moves slowly
across the background of stars until it finally fades away.

People used to think that a comet's sudden appearance in the sky
foretold a terrible event on Earth. Later, astronomers discovered
that comets are harmless bodies that travel in ellipses around the
Sun.

Comets do not last for ever. Each time one passes the Sun it
loses some of its gas until, eventually, only the dust particles are
left. Sometimes the Earth crosses the path of a swarm of dust
particles from a former comet which plunge through the Earth's
atmosphere as 'shooting stars'. (See Meteors.)

NASA solar system facts:

although comets might be thought to be
bright objects that flash across the sky, they are usually dim and
move very slowly. For this reason long exposures are necessary to
photograph them, and the background of stars may appear as lines as
the camera follows the comet's motion across the sky

NASA solar system facts: comet Alcock appeared in the sky in
1959. Below: The most famous comet of all, Halley's comet, appears
every 76 years. It was last seen in 1910 and is expected again in
1986. It was seen as an omen before the Battle of Hastings in 1066.